Verify your iCloud Push Settings
On your iOS device
Open the Settings app
Tap Calendar, then tap Accounts (iOS 14). For <= iOS 13, tap Passwords & Accounts instead
Tap Fetch New Data
Make sure Push is turned on at the top
Tap the iCloud entry below
Choose Push in the SELECT SCHEDULE section
Turn iCloud Push ON and OFF
Once in a while, your phone may experience issues with iCloud Push, and turning it on and off will often fix the issue.
On your iOS device
Open the Settings app
Tap Calendar, then tap Accounts (iOS 14). For <= iOS 13, tap Passwords & Accounts instead
Tap Fetch New Data
Turn OFF Push at the top
Wait one minute
Turn ON Push at the top
Open the Calendar app on your iOS device